Epoxy adhesives are widely used to form permanent bonds between substrates. High modulus epoxy adhesives are often used when it is desired to form a bond having high overlap shear strength and high T-peel strength.
When substrates having unequal or mismatched coefficients of thermal expansion are adhesively bonded to one another, the unequal thermal expansion rates of the substrates may cause stress to build up in the adhesive and/or in the substrate. For example, when adhesively bonded substrates are exposed to changing temperature, the stress built up in the adhesive and/or the substrate may cause the adhesive bond to prematurely fail.
One approach to bonding substrates having unequal coefficients of thermal expansion is to utilize highly flexible adhesives, for example, epoxy adhesives cured with long chain amines. These types of adhesives have a polymeric structure that allows the adhesive to relieve stress that may be imparted by movement of one or more of the substrates. Although these adhesives may be suitable for some bonding applications, they are typically quite soft and have a large shear creep under load. That is, the adhesive may allow the bonded substrates to move relative to one another when subjected to shear load.
Certain demanding bonding applications require adhesives that can both bond substrates having unequal or mismatched coefficients of thermal expansion, and that have a low shear creep under load. For example, certain optical devices require the bonding of glass to plastic in such a manner that the bond can withstand repeated cycling of the bonded substrates from low temperature (e.g., −40° C.) to high temperature (e.g., 60° C.) with no spalling of the glass substrate. In addition, the adhesive must have a shear creep under load of about 10 microns or less in order to prevent unacceptable loss of alignment of the optical components in the device.
In view of the foregoing, what is desired is an adhesive that can bond substrates having unequal or mismatched substrates and which has a low shear creep under load.